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Perimeter researcher Michal P. Heller has created a shortcut to understand primordial matter in its most extreme state.

To recreate the birth of the universe, most physicists believe that you would first need a big bang. This would be followed almost instantly by the appearance of an ultra-hot blob of plasma, the primordial soup that formed the basis of everything in existence.

Quark-gluon plasma

It’s not exactly a process that is easily replicated, but in heavy-ion colliders, experimentalists are creating “little bangs” to produce tiny droplets of quark-gluon plasma (QGP).

The experiments offer a glimpse of the kind of matter that filled the early universe, but there’s a problem. In its earliest stages – between “bang” and “goop” – the system is in extreme distortion. In fact, it’s not until the system cools down that it reaches equilibrium and can be assessed.

It is this period of ultra-hot chaos that intrigues Perimeter Institute postdoctoral researcher Michal P. Heller. And in a new paper published recently in Physical Review Letters, he has brought together two seemingly different fields of study – hydrodynamics and string theory – to help describe it.

In ordinary matter, quarks do not exist in isolation; they are always bound extremely tightly by gluons in atomic nuclei. But in extreme conditions – say, in temperatures a million times hotter than the sun – these bonds can “melt,” forming an ultra-hot, almost-frictionless plasma in which quarks and gluons move freely.

The tiny droplets produced by these “little bangs” exist for brief flashes of time, like super-hot fireballs that quickly expand and cool into ordinary matter. In the moment after the collision, the ultra-hot system is in extreme distortion, existing at the threshold of being called a fluid.

Despite the fact that theorists know the microscopic rules that govern such ultra-energetic collisions, questions still abound. It is clear these systems only become plasma once they settle enough to reach equilibrium. Working out what happens in the chaotic interim, before the system equilibrates, requires extremely complex computations.

The standard modelling for plasma experiments uses relativistic hydrodynamics, a theory similar to that describing the motion of water but which also incorporates [Albert] Einstein’s special relativity. (This is because QGP and its microscopic constituents move with large velocities, at which relativistic effects become important.)

Some researchers, including Heller, have simplified the problem by equating the relaxation of the QGP “fireball” to a black hole reaching equilibrium in a hypothetical five-dimensional space. This approach takes methods derived from string theory, and applies them to the physics of the “little bang” experiments and the droplets of QGP they create.

Now, Heller and his co-authors have put forward a computational technique that is something of a further short-cut for theorists: instead of doing the calculations using five-dimensional Einstein equations – which is very complicated – they have developed a way to incorporate part of those calculations into a four-dimensional description that is coupled to conventional hydrodynamics equations.

“The paper shows something about the theories of relativistic hydrodynamics that was known, but not many people had thought about it seriously,” Heller says. “Our observation opens up a new possibility of describing transient relaxation effects governing the approach to the quark-gluon plasma phase.”

This work is a return of sorts for Heller, a Polish scientist who came to Perimeter in 2014 from the University of Amsterdam. His research career began with a 2007 paper studying theories of second-order relativistic hydrodynamics, which factors causal evolution into standard fluid dynamics. In 2012 and 2013, his work in string theory and strong gravity brought him back to those theories, but with a new perspective.

“What’s been fun is coming back to the project I started my research career with, and realizing that the things which I thought several years ago were simple are actually not so trivial and have far-reaching consequences,” he says.

When he was younger, he thought the universe could be understood through one simple model. Now, he sees much more nuance: “At some point, you start appreciating that everything is complex and interconnected.”

QGP is like that, too, he says. While researchers would probably like to create QGP in a simple state of equilibrium (so that they can introduce their own distortions and measure the effects), reality is much more complex. Current experimental and theoretical approaches aren’t sensitive enough to capture and analyze in detail the droplets’ initial, highly distorted state, but Heller’s paper is a step in this direction.

“Is that a choice? It’s more of a necessity, at least given what we have available here on Earth,” he says.

Should other researchers build on this work, combining it with complementary approaches of initial state physics to construct some sort of a hybrid, he says these generalized theories of hydrodynamics “will be a crucial ingredient of whatever comes next.”

Perimeter Institute is a leading centre for scientific research, training and educational outreach in foundational theoretical physics. Founded in 1999 in Waterloo, Ontario, Canada, its mission is to advance our understanding of the universe at the most fundamental level, stimulating the breakthroughs that could transform our future. Perimeter also trains the next generation of physicists through innovative programs, and shares the excitement and wonder of science with students, teachers and the general public.

One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world.Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.

By combining data from two high-energy accelerators, nuclear scientists have refined the measurement of a remarkable property of exotic matter known as quark-gluon plasma. The findings reveal new aspects of the ultra-hot, “perfect fluid” that give clues to the state of the young universe just microseconds after the big bang.

The multi-institutional team known as the JET Collaboration, led by researchers at the U.S. Department of Energy’s Lawrence Berkeley National Lab (Berkeley Lab), published their results in a recent issue of Physical Review C. The JET Collaboration is one of the Topical Collaborations in nuclear theory established by the DOE Office of Science in 2010. JET, which stands for Quantitative Jet and Electromagnetic Tomography, aims to study the probes used to investigate high-energy, heavy-ion collisions. The Collaboration currently has 12 participating institutions with Berkeley Lab as the leading institute.

“We have made, by far, the most precise extraction to date of a key property of the quark-gluon plasma, which reveals the microscopic structure of this almost perfect liquid,” says Xin-Nian Wang, physicist in the Nuclear Science Division at Berkeley Lab and managing principal investigator of the JET Collaboration. Perfect liquids, Wang explains, have the lowest viscosity-to-density ratio allowed by quantum mechanics, which means they essentially flow without friction.

Inside protons and neutrons that make up the colliding atomic nuclei are elementary particles called quarks, which are bound together tightly by other elementary particles called gluons. Only under extreme conditions, such as collisions in which temperatures exceed by a million times those at the center of the sun, do quarks and gluons pull apart to become the ultra-hot, frictionless perfect fluid known as quark-gluon plasma.

“The temperature is so high that the boundaries between different nuclei disappear so everything becomes a hot-plasma soup of quarks and gluons,” says Wang. This ultra-hot soup is contained within a chamber in the particle accelerator, but it is short-lived—quickly cooling and expanding—making it a challenge to measure. Experimentalists have developed sophisticated tools to overcome the challenge, but translating experimental observations into precise quantitative understanding of the quark-gluon plasma has been difficult to achieve until now, he says.

Looking Inside

In this new work, Wang’s team refined a probe that makes use of a phenomenon researchers at Berkeley Lab first theoretically outlined 20 years ago: energy loss of a high-energy particle, called a jet, inside the quark gluon plasma.

“When a hot quark-gluon plasma is generated, sometimes you also produce these very energetic particles with an energy a thousand times larger than that of the rest of the matter,” says Wang. This jet propagates through the plasma, scatters, and loses energy on its way out.

Since the researchers know the energy of the jet when it is produced, and can measure its energy coming out, they can calculate its energy loss, which provides clues to the density of the plasma and the strength of its interaction with the jet. “It’s like an x-ray going through a body so you can see inside,” says Wang.

Xin Nian Wang, physicist in the Nuclear Science Division at Berkeley Lab and managing principal investigator of the JET Collaboration.

One difficulty in using a jet as an x-ray of the quark-gluon plasma is the fact that a quark-gluon plasma is a rapidly expanding ball of fire—it doesn’t sit still. “You create this hot fireball that expands very fast as it cools down quickly to ordinary matter,” Wang says. So it’s important to develop a model to accurately describe the expansion of plasma, he says. The model must rely on a branch of theory called relativistic hydrodynamics in which the motion of fluids is described by equations from Einstein’s theory of special relativity.

Over the past few years, researchers from the JET Collaboration have developed such a model that can describe the process of expansion and the observed phenomena of an ultra-hot perfect fluid. “This allows us to understand how a jet propagates through this dynamic fireball,” says Wang

Employing this model for the quark-gluon plasma expansion and jet propagation, the researchers analyzed combined data from the PHENIX and STAR experiments at RHIC and the ALICE and CMS experiments at LHC since each accelerator created quark-gluon plasma at different initial temperatures. The team determined one particular property of the quark-gluon plasma, called the jet transport coefficient, which characterizes the strength of interaction between the jet and the ultra-hot matter. “The determined values of the jet transport coefficient can help to shed light on why the ultra-hot matter is the most ideal liquid the universe has ever seen,” Wang says.

PHENIX at BNL

STAR at BNL

ALICE at CERN

CMS at CERN

Peter Jacobs, head of the experimental group at Berkeley Lab that carried out the first jet and flow measurements with the STAR Collaboration at RHIC, says the new result is “very valuable as a window into the precise nature of the quark gluon plasma. The approach taken by the JET Collaboration to achieve it, by combining efforts of several groups of theorists and experimentalists, shows how to make other precise measurements of properties of the quark gluon plasma in the future.”

The team’s next steps are to analyze future data at lower RHIC energies and higher LHC energies to see how these temperatures might affect the plasma’s behavior, especially near the phase transition between ordinary matter and the exotic matter of the quark-gluon plasma.

Using a calculation originally proposed seven years ago to be performed on a petaflop computer, Lawrence Livermore researchers computed conditions that simulate the birth of the universe.

When the universe was less than one microsecond old and more than one trillion degrees, it transformed from a plasma of quarks and gluons into bound states of quarks – also known as protons and neutrons, the fundamental building blocks of ordinary matter that make up most of the visible universe.

In a paper appearing in the Aug. 18 edition of Physical Review Letters, Lawrence Livermore scientists Chris Schroeder, Ron Soltz and Pavlos Vranas calculated the properties of the QCD phase transition using LLNL’s Vulcan, a five-petaflop machine. This work was done within the LLNL-led HotQCD Collaboration, involving Los Alamos National Laboratory, Institute for Nuclear Theory, Columbia University, Central China Normal University, Brookhaven National Laboratory and Universität Bielefed in Germany.

A five Petaflop IBM Blue Gene/Q supercomputer named Vulcan

This is the first time that this calculation has been performed in a way that preserves a certain fundamental symmetry of the QCD, in which the right and left-handed quarks (scientists call this chirality) can be interchanged without altering the equations. These important symmetries are easy to describe, but they are computationally very challenging to implement.

“But with the invention of petaflop computing, we were able to calculate the properties with a theory proposed years ago when petaflop-scale computers weren’t even around yet,” Soltz said.

The research has implications for our understanding of the evolution of the universe during the first microsecond after the Big Bang, when the universe expanded and cooled to a temperature below 10 trillion degrees.

Below this temperature, quarks and gluons are confined, existing only in hadronic bound states such as the familiar proton and neutron. Above this temperature, these bound states cease to exist and quarks and gluons instead form plasma, which is strongly coupled near the transition and coupled more and more weakly as the temperature increases.

“The result provides an important validation of our understanding of the strong interaction at high temperatures, and aids us in our interpretation of data collected at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory and the Large Hadron Collider at CERN.” Soltz said.

RHIC at Brookhaven

LHC at CERN

Soltz and Pavlos Vranas, along with former colleague Thomas Luu, wrote an essay predicting that if there were powerful enough computers, the QCD phase transition could be calculated. The essay was published in Computing in Science & Engineering in 2007, “back when a petaflop really did seem like a lot of computing,” Soltz said. “With the invention of petaflop computers, the calculation took us several months to complete, but the 2007 estimate turned out to be pretty close.”

The extremely computationally intensive calculation was made possible through a Grand Challenge allocation of time on the Vulcan Blue Gene/Q Supercomputer at Lawrence Livermore National Laboratory.

New analyses of deuteron-gold collisions at RHIC reveal that even small particles can create big surprises

December 6, 2013
Karen McNulty Walsh

Scientists designed and built the Relativistic Heavy Ion Collider (RHIC) at the U.S. Department of Energy’s Brookhaven National Laboratory to create and study a form of matter that last existed a fraction of a second after the Big Bang, some 13.8 billion years ago. The early-universe matter is created when two beams of gold nuclei traveling close to the speed of light slam into one another. The high-speed particle smashups pack so much energy into such a tiny space that the hundreds of protons and neutrons making up the nuclei “melt” and release their constituent particles—quarks and gluons—so scientists can study these building blocks of matter as they existed at the dawn of time.

Components of the PHENIX detector at Brookhaven’s Relativistic Heavy Ion Collider (RHIC). PHENIX weighs 4,000 tons. It has large steel magnets and a dozen detector sub-systems that bend and track a wide range of particles while measuring their properties (e.g., momentum and energy) as they emerge from collisions. No image credit.

Collisions between gold nuclei and deuterons—much smaller particles made of just one proton and one neutron—weren’t supposed to create this superhot subatomic soup known as quark-gluon plasma (QGP). They were designed as a control experiment, to generate data to compare against RHIC’s gold-gold smashups. But new analyses indicate that these smaller particle impacts may be serving up miniscule drops of hot QGP—a finding consistent with similar results from Europe’s Large Hadron Collider (LHC), which can also collide heavy nuclei.

“Considering that the quark-gluon plasma we create in gold-gold collisions at RHIC fills a space that is approximately the size of the nucleus of a single gold atom, the possible hot spots we’re talking about in these deuteron-gold collisions are much, much smaller—and an intriguing surprise,” said Dave Morrison, a physicist at Brookhaven and co-spokesperson for RHIC’s PHENIX collaboration. The collaboration describes their results in two papers just published by Physical Review Letters, one of which is highlighted by the journal.

The findings at RHIC and the LHC have triggered active debate about their interpretation. Said PHENIX co-spokesperson Jamie Nagle of the University of Colorado, “There isn’t yet universal agreement about what we’re seeing in these small systems, but if indeed nearly perfect fluid droplets of quark-gluon plasma are being formed, this may be a perfect testing ground for understanding the essential conditions for creating this remarkable state of matter.”

One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world.Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.

THERE IS A TON OF UNEXPLAINED JARGON IN THIS ARTICLE. IT IS REALLY WRITTEN EXPECTING SPECIALISTS TO BE THE READERS. BUT I OFFER IT FOR ANYONE WHO MIGHT KNOW WHAT IS GOING ON OR WHO MIGHT WISH A STARTING POINT TO DIG IN AND FIND OUT.

21 March 2013
Panos Charitos

“20 years ago ALICE started its amazing adventure in the wonderland of strong interactions and the study of extraordinary forms of matter like the Quark Gluon Plasma.

CERN’s ion programme has a long history and was initiated in 1986 with the acceleration of oxygen ions at 60 and 200 GeV/nucleon, and continued with sulphur ions at 200 GeV/nucleon up to 1993. The first Lead-ion beams at 160 GeV/nucleon became available in 1994. The accelerating chain for 16O and 32S consisted of an ion source of the electron–cyclotron resonance (ECR) type, a radio-frequency quadrupole (RFQ) pre-accelerator, the linear accelerator injector (LINAC I), the PSBooster , the PSand the SPS. For the acceleration of lead ions, a new ECR source, a new RFQ and a new LINAC had to be constructed. The results of the light-ion programme strongly supported its continuation with heavier-ion beams. In particular, the energy densities reached during the collisions appeared to be high enough to be interesting, and many of the suggested signatures for the onset of a quark–gluon plasma phase turned out to be experimentally accessible. The experience gained was instrumental in assessing the feasibility of experiments with lead ions and for indicating the necessary detector modifications. Seven experiments participated in the lead-age adventure.

Following the previous successes of the heavy-ion physics programme at CERN the idea of a heavy-ion dedicated experiment that would study lead-lead collisions at the new energy scale of the LHC was discussed. During the previous years, the experience gained was instrumental in assessing the feasibility of experiments with lead ions and for indicating the necessary detector modifications that were needed to move with the lead-age adventure at the new scale of the LHC.

The first appearance of ALICE was in the Evian meeting back in 1992. Jurgen Schukraft recalls that: “We had to do enormous extrapolations because the LHC was a factor of 300 higher in centre-of-mass energy and a factor of 7 in beam mass compared with the light-ion programme, which started in 1986 at both the CERN SPS and the Brookhaven AGS.” A Letter of Intent for a new experiment at the LHC was submitted on 1 March 1993 to the LHC Committee that was formed shortly after the Evian meeting. It marks the first official use of the name ALICE and it was signed by 230 people coming from 42 institutes around the world. It was clearly describing the proposal of the ALICE Collaboration for building a dedicated heavy-ion detector to exploit the unique physics potential of nucleus-nucleus interaction at LHC energies and where the formation of a new phase of matter, the quark gluon plasma is expected. The submission of the letter of intent was followed by a detailed technical proposal that was submitted two years later in 1995 and shortly endorsed by the LHCC and the CERN management.

ALICE studies strong interactions by using particles – created inside the hot volume of the Quark Gluon Plasma as it expands and cools down – that live long enough to reach the sensitive detector layers located around the interaction region. The physics programme at ALICE relies on being able to identify all of them – i.e. to determine if they are electrons, photons, pions, etc – and to determine their charge. This involves making the most of the different ways that particles interact with matter. Over twenty years, ALICE has developed a wide range of R&D activities, confronted many challenges in designing and building new detectors that could cope with the physical challenges at the new energy scales. One should also refer to the big data challenge as heavy-ion collisions produce petabytes of data that need to be stored and later analysed in order to get new physics results.

Following the first run, ALICE successfully reported on the formation of QGP and offered a new insight on the nature of strong interacting matter at extreme densities. The existence of such a phase and its properties are a key issue in QCD for the understanding of confinement and of chiral-symmetry restoration. Wherever you look, from the energy loss of fast quarks to quarkonia, from the details of the dynamical evolution of the system to the very first study of charmed hadrons and the loss of energy, the interplay between , to name just a few, the ALICE results stand out for their quality and relevance. Following the recent proton-lead run that opens new horizons for the heavy-ion community at CERN, ALICE is now looking forward to a series of upgrades during the LS1.

Paolo Giubellino notes: ‘This has been the result of many years of work and dedication of all of us, and we can all be proud of now sharing this remarkable harvest. It has been an enormous effort, but we can now say it was really worth it, and all share the happiness for this wealth of results. We all contributed to this accomplishment, and we should all draw from it even more motivation to go forward for the next many years to come!'”

Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

Friday, Jan. 18, 2013
Jim Pivarski

“In most particle physics experiments, physicists attempt to concentrate as much energy as possible into a point of space. This allows the formation of new, exotic particles like Higgs bosons that reveal the basic workings of the universe. Other collider experiments have a different goal: to spread the energy among enough particles to make a continuous medium, a droplet of fluid millions of times hotter than the center of the sun. The latter studies, often referred to as heavy-ion physics, require collisions of large nuclei, such as gold or lead, to produce amorphous splashes instead of point-like collisions.

Quark-gluon Plasma at Brookhaven’s RHIC (image NSCL)

This short-lived state of quark matter is unlike any other known to science. All other liquids, gases, gels and plasmas are governed by forces that weaken with distance… In contrast, the quarks and gluons loosed by a heavy-ion collision are attracted to one another by the nuclear strong force, which does not weaken with distance. As two quarks start to separate from each other, new pairs of quarks and antiquarks join the mix with an attraction of their own.

Quark matter is the stuff the big bang was made of. In the first microseconds of the universe, all matter was a freely flowing quark-gluon soup [*], which later evaporated into the protons and neutrons that we know today. Yet it is far from understood.

Heavy-ion collisions in the LHC and RHIC at Brookhaven will tell us more about the origin of our universe.

Relativistic Heavy Ion Collier (RHIC) at Brookhaven

CMS – the home of QGP research at CERN’s LHC
See the full article here.

“The ALICE experiment is dedicated to the study of the quark-gluon plasma. Each year, the LHC operates for a few weeks with lead ions instead of protons. ALICE collects data both during proton-proton collisions and heavy ions collisions. Even when only protons collide, the projectiles are not solid balls like on a billiard table but composite objects. By comparing what can is obtained from heavy ion collisions with proton collisions, the ALICE physicists must first disentangle what comes from having protons in a bound state inside the nucleus as opposed to “free protons”.

So far, it appears that the quark-gluon plasma only formed during heavy-ion collisions since they provide the necessary energy density over a substantial volume (namely, the size of a nucleus). Some of the effects observed, such as the number of particles coming out of the collisions at different angles or momenta, depend in part on the final state created. When the plasma is formed, it reabsorbs many of the particles created, such that fewer particles emerged from the collision.

By colliding protons and heavy ions, scientists hope to discern what comes from the initial state of the projectile (bound or free protons) and what is caused by the final state (like the suppression of particles emitted when a quark-gluon plasma forms).

A “snapshot” of the debris coming out of a proton-lead ion collision captured by the ALICE detector showing a large number of various particles created from the energy released by the collision.

The ultimate goal is to study the so-called ‘structure function’, which describes how quarks and gluons are distributed inside protons, when they are free or embedded inside the nucleus.

More will be studied during the two-month running period with protons colliding on heavy ions planned for the beginning of 2013.”